PENDLETON — A low pressure system off the Pacific Coast is expected to bring relief at week’s end from the blanket of smoke smothering much of Oregon and Washington.

Walt Clark, a forecaster at Pendleton’s National Weather Service Office, said it looks like Umatilla County residents probably wouldn’t be seeing patches of blue sky until Friday afternoon, Sept. 18.

“Already on the coast today things look excellent, but it will take a little while to work inland toward the Columbia Basin,” he said.

Until then, most Oregonians should limit their outdoor exposure when possible.

“The basin and valleys are entrenched under an inversion,” Roger Cloutier, lead forecaster at Pendleton’s National Weather Service Office, said. ”The winds aloft are not mixing down into lower elevations, so the smoke is trapped.”

The flow aloft is becoming southwesterly, Cloutier said, and is mixing down to higher ridges and peaks, allowing smoke to disperse, especially in Redmond, Bend, Prineville and even the higher elevations of the Blue Mountains and the Wallowas.

According to air monitoring data provided by the Oregon Department of Environmental Quality, the Air Quality Index, which measures the amount of pollutants in the air, recommends cutting back on outdoor activities. People with heart or lung disease, older adults, children and teens should avoid physical activities outdoors.

On the morning of Wednesday, Sept. 16, the Air Quality Index on airnow.gov. listed Hermiston’s and Pendleton’s air quality as “very unhealthy,” downgraded from “hazardous” earlier in the week.

Moving east, La Grande, Enterprise and Baker city were downgraded to “unhealthy.”

A large, low pressure system sitting over the Pacific will move inland Thursday, Sept. 17, and Sept. 18 with a deep, southerly flow, Cloutier said, which will bring instability and moisture into Oregon and Washington.

“As the system shifts eastward the smoke will disperse, and bring a slight chance of thunderstorms to develop,” he said.

The prediction is for more showers than thunderstorms, Cloutier said, with winds coming from the south, shifting to the west and later the northwest.

“Whatever does form the system will be wet and hopefully will be enough to scour out the smoke,” he said.

Clark reminded residents that even if the smoke clears over the weekend, it could return again the following week, depending on fire conditions.